1. Field of the Invention
The present invention relates to a piston and more particularly to a piston for an internal combustion engine.
2. Description of the Related Art
A conventional piston designed to reduce piston slap and prevent the seizure of a skirt part or a ring is disclosed in Japanese Patent Publication No. 58(1983)-25861, and is shown in FIGS. 5, 6. The piston 30 comprises a land part 31 and a skirt part 32. The diameter of the land part 31 is smaller than the diameter of the lower portion of the skirt part 32. The diameters of the skirt part 32 to each side of the axis of pin hole 33 (i.e., spaced vertically from the axis of the pin hole as oriented in FIG. 5) is smaller than the diameter on line C--C, which is perpendicular to the axis of pin hole 33. The piston 30 has an oval (i.e., elliptical) shape in the horizontal cross-section (as oriented in FIG. 5) in both the land and skirt parts 31, 32. The axis B--B of the land part 31 is offset from the axis A--A of the skirt part 32 about 30.about.60 .mu.m; that is, the center of the ovality of the land 31 is offset to the thrust side from the center of the ovality of the skirt part 32. Hence, at the slap (i.e., when the under part or lower portion of the skirt part 32 hits the bore sidewall after the upper dead point by the explosion) the head of the piston 30 slants to the anti-thrust side and reduces the clearance of the land part 31 on the thrust side. Therefore, the reduction of the blowby gas and the seizure of the land part 31 (and the ring) or the skirt part 32 as a result of the reduction of the temperature are prevented.
However, at the piston slap (especially at the upper dead point by the explosion) the set position of the piston 30 is changed by the offset of the pin. The more the pin hole 33 is offset to the thrust side, and the more the piston is biased to the anti-thrust side from the under part or lower portion of the skirt part 32, the more the skirt 32 hits the bore on the thrust side, and the more the hit point moves to the upper side of the cylinder 34. If the ovality of the land part 31 is offset to the thrust side as compared to the skirt part 32, the pin hole 33 has the effect of being biased in the thrust offset amount of the land part 34 under the expansion of the piston 30. As shown in FIG. 6, the shoulder 31a of the land part 31 and the shoulder 32a of the skirt part 32 hit the sidewall of the bore hard. Thus, even if the pin hole 33 is biased more to the thrust side and the skirt part 32 hits the bore of the thrust side along the under part in order to cancel the effect of the offset to the anti-thrust side, the piston 30 still rolls and hits the bore sidewall in a certain range of its own from the under part to the upper part of the skirt. Therefore, it is unavoidable, that the shoulders 31a and 32a of the land and the shoulder hit the bore of the cylinder 34 hard. Furthermore, because the temperature of the land part 31 is higher than that of the skirt part 32 under the driving of the engine, the heat expansion of the land part 31 is more than that of the skirt part 32 and the offset side of the land part 31 hits the side of the bore harder. Therefore, contrary of the description of the prior art, the prior art unsatisfactorily prevents the slap and seize of the piston.